3694(RevA) Просмотр технического описания (PDF) - Linear Technology

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производитель

3694
(Rev.:RevA)

36V, 2.6A Monolithic Buck Regulator With Dual LDO

Linear Technology 

LT3694/LT3694-1

APPLICATIONS INFORMATION

STEP DOWN SWITCHING REGULATOR

Feedback Resistor Network

The output voltage is programmed with a resistor divider

(refer to the Block Diagram in Figure 1) between the output

and the FB pin. Choose the resistors according to:

R1=

R2

⎛

⎝⎜

VOUT

750mV

−

1⎞⎠⎟

The parallel combination of R1 and R2 should be 10k or

less to avoid bias current errors.

Input Overvoltage Lockout

An important feature of the LT3694 is the ability to survive

transient surges on the input voltage of up to 70V. This is

accomplished by shutting off the regulators to keep this

high voltage off the critical components. The overvoltage

lockout trips when the input voltage exceeds 38V.

Input Voltage Range

The minimum operating voltage is determined either by the

LT3694’s internal undervoltage lockout or by its maximum

duty cycle. The duty cycle is the fraction of time that the

internal switch is on and is determined by the input and

output voltage:

DC = VOUT + VF

VIN − VSW + VF

where VF is the forward voltage drop of the catch diode

and VSW is the voltage drop of the internal switch (~0.3V

at maximum load). This leads to a minimum input

voltage of:

VIN(MINCF )

=

VOUT + VF

DCMAX(CF )

−

VF

+

VSW

The duty cycle is the fraction of time that the internal

switch is on during a clock cycle. The maximum duty cycle

for constant-frequency operation given by DCMAX(CF) = 1

– tOFF(MIN) • fSW. However, unlike most fixed frequency

regulators, the LT3694 will not switch off at the end of

each clock cycle if there is sufficient voltage across the

boost capacitor (C3 in Figure 1) to fully saturate the output

switch. A forced switch off for a minimum time will only

occur at the end of a clock cycle when the boost capaci-

tor needs to be recharged. This operation has the same

effect as lowering the clock frequency for a fixed off time,

resulting in a higher duty cycle and lower minimum input

voltage. The resultant duty cycle depends on the charging

times of the boost capacitor and can be approximated by

the following equation:

DCMAX

=

B

B+

1

where B is the output current divided by the typical

boost current from the BST Pin Current vs Switch Cur-

rent curve in the Typical Performance Characteristics

section.

The maximum voltage, VIN, for constant-frequency opera-

tion is determined by the minimum duty cycle DCMIN:

VIN(MAXCF )

=

VOUT + VF

DCMIN

−

VF

+

VSW

with DCMIN = tON(MIN) • fSW

Thus, both the maximum and minimum input voltages

for constant-frequency operation are a function of the

switching frequency and output voltage. Therefore, the

maximum switching frequency must be set to a value that

accommodates the input and output voltage parameters

and must meet both of the following criteria:

fMAX1=

⎛

⎜

⎝

VOUT + VF

VIN(MAXCF) − VSW

+

VF

⎞

⎟

⎠

•

1

tON(MIN)

fMAX 2

=

⎛

⎜ 1−

⎝

VOUT + VF

VIN(MINCF) − VSW

+

VF

⎞

⎟

⎠

•

1

tOFF(MIN)

The values of tON(MIN) and tOFF(MIN) are functions of

ISW and temperature (see chart in the Typical Perform-

ance Characteristics section). Worst-case values for

switch currents greater than 0.5A are tON(MIN) = 130ns and

36941fa

10

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